Requirements for an Ideal Rail Section

(a) The rail should have the most economical section consistent with strength, stiffness, and durability.

(b) The centre of gravity of the rail section should preferably be very close to the mid-height of the rail so that the maximum tensile and compressive stresses are equal.

(c) A rail primarily consists of a head, a web, and a foot, and there should be an economical and balanced distribution of metal in its various components so that each of them can fulfil its requirements properly. The requirements, as well as the main considerations, for the design of these rail components are as follows.

Head: The head of the rail should have adequate depth to allow for vertical wear. The rail head should also be sufficiently wide so that not only is a wider running surface available, but also the rail has the desired lateral stiffness. Web: The web should be sufficiently thick so as to withstand the stresses arising due to the loads bone by it, after allowing for normal corrosion. Foot: The foot should be of sufficient thickness to be able to withstand vertical and horizontal forces after allowing for loss due to corrosion. The foot should be wide enough for stability against overturning. The design of the foot should be such that it can be economically and efficiently rolled. Fishing angles: Fishing angles must ensure proper transmission of loads from the rails to the fish plates. The fishing angles should be such that the tightening of the plate does not produce any excessive stress on the web of the rail.

Height of the rail: The height of the rail should be adequate so that the rail has sufficient vertical stiffness and strength as a beam.

Standard Rail Section

The rail is designated by its weight per unit length. In FPS units, it is the weight in lbs per yard and in metric units it is in kg per metre. A 52 kg/m rail denotes that it has a weight of 52 kg per metre.

The weight of a rail and its section is decided after considerations such as the following:

(a) Heaviest axle load

(b) Maximum permissible speed

(c) Depth of ballast cushion

(d) Type and spacing of sleepers

(e) Other miscellaneous factors

The standard rail sections in use on Indian Railways are 60 kg, 52 kg, 90 R, 75 R, 60 R and 50 R. The two heavier rail sections, 60 kg and 52 kg, were recently introduced and are designated in metric units. Other rails are designed as per the revised British Standard specifications and are designated in FPS units though their dimensions and weight are now in metric units. In the nomenclature 90 R, 75 R, etc., R stands for revised British specifications.

Every rail rolled has a brand on its web, which is repeated at intervals. As per IRS-T-12-88, the brand marks are as follows:

It may be mentioned here that the 90 R rail section is adequate only for an annual traffic density of about 10 GMT (gross million tonnes per km/annum), speeds of up to 100 kmph, axle loads up to main line (ML) standard, and a service life of about 20-25 years. Realizing to these limitations, the Indian Railways, in the year 1959, designed a heavier rail section of 52 kg/m to meet the requirements of heavier and faster traffic. This rail section was recommended for use on all BG main line routes with future speeds of up to 130 kmph and traffic density of 20-25 GMT. The important dimensions of 52-kg and 60-kg rails are shown in Fig. 6.5.

The traffic density on the BG track routes of Indian Railways is increasing very fast. Accordingly, to meet the future requirements of traffic, a new design has been finalized for the 60-kg UIC section rail. The rail section has been designed for speeds of up to 160 kmph and a traffic density of about 35 GMT.

Weight of rails

Though the weights of a rail and its section depend upon various considerations, the heaviest axle load that the rail has to carry plays the most important role. The following is the thumb rule for defining the maximum axle load with relation to the rail section:

Theoretically, the longer the rail, the lesser the number ofjoints and fittings required and the lesser the cost of construction and maintenance. Longer rails are economical and provide smooth and comfortable rides. The length of a rail is, however, restricted due to the following factors.

(a) Lack of facilities for transport of longer rails, particularly on curves.

(b) Difficulties in manufacturing very long rails.

(c) Difficulties in acquiring bigger expansion joints for long rails.

(d) Heavy internal thermal stresses in long rails.

Taking the above factors into consideration, Indian Railways has standardized a rail length of 13 m (previously 42 ft) for broad gauge and 12 m (previously 39 ft) for MG and NG tracks. Indian Railways is also planning to use 26 m, and even longer, rails in its track system.